Linear accelerators, or Linacs, are devices which use radio frequency energy to accelerate charged particles such as electrons, protons, and ions. Charged particles from an ion source enter a cylindrical enclosure known as a tank which encloses coaxially spaced devices known as drift tubes. RF energy is present in the tank for accelerating the charged particles through the gaps between adjacent drift tubes. The construction of a linear accelerator is such that the particles become shielded within each drift tube from the effects of RF voltage reversals. Thus, as charged particles emerge from each drift tube and enter the next gap, they become further accelerated. It is necessary to provide means for focussing the charged particle beam along the axis of the tank so as to counteract its tendency to diverge. Different types of magnets are employed to do this and in co-pending U.S. Pat. applications Ser. No. 07/522,825 filed May 14, 1990, and U.S. Pat. application Ser. No. 07/507,768 filed Apr. 12, 1990, the utilization of quadrupole permanent magnets is discussed. These magnets are fabricated from rare earth cobalt segments.
A drift tube includes a central hollow cylindrical body having an annular permanent magnet assembly mounted therein. The interior of the drift tube is sealed by face plates which form transverse boundaries for gaps between adjacently positioned drift tubes. A manufacturing problem arises due to the thermal sensitivity of the permanent magnet assembly. Particularly, with rare earth cobalt permanent magnets, it is important to avoid exposure of the magnet assembly to temperatures over 100.degree. C. Above this temperature the quadrupole magnetic properties become diminished.
With present drift tube assemblies attempts are usually made to perform electron beam welding of the face plates. However, this requires careful control due to the thermal sensitivity of the magnet assembly. Further, it is difficult to use electron beam welding since the electron beam becomes influenced by the strong magnetic field of the magnet assembly.
Japanese investigators at the National Laboratory for High Energy Physics in Japan and the Institute for Nuclear Study at the University of Tokyo have proposed the utilization of copper electroplating to seal the drift tube face plates. An electroplating procedure is extremely desirable due to the fact that the process is performed at room temperature. However, the attempt to electroplate a seal over the joint created between the face plates and the central body of the drift tube, as proposed by these investigators, is incomplete without thorough disclosure of preliminary joint seal steps which must be taken to ensure that no leakage of electroplating bath liquid (typically sulfuric acid) into the drift tube occurs. A preliminary joint seal must also create a continuous electrical and thermal interface, capable of remaining so after extreme thermal cycling. The creation of a gap in the joint would severely affect the cooling capability of the drift tube.